Variable wavelength dispersion compensator

ABSTRACT

Beams inputted from a fiber are collected by a lens and are angular-dispersed by a VIPA. The luminous flux from the VIPA is collected on a surface-shape variable mirror by a lens. The surface-shape variable mirror is configured in such a way that a mirror shape can be controlled by a piezo stage and necessary wavelength dispersion can be applied, if necessary. Although the beam group reflected on the surface-shape variable mirror propagates the light path backward, the beam group is inputted to a position different from the outputted position when the beam group enters the VIPA. Therefore, a desired wavelength dispersion can be given to each beam group by performing control of the input position in the VIPA for each wavelength using the surface-shape variable mirror.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of application Ser. No.09/986,293, filed Nov. 8, 2001, and now allowed.

This application is based upon and claims the priority of Japaneseapplication no. 2001-216415, filed Jul. 17, 2001, and U.S. patentapplication Ser. No. 09/986,293, filed Nov. 8, 2001, the contents beingincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a variable wavelength dispersioncompensator for variably compensating wavelength dispersion in anoptical fiber communications system.

2. Description of the Related Art

An optical fiber communications system generally has a problem that thedistortion of a transmission waveform due to optical fiber wavelengthdispersion (chromatic dispersion) degrades signal quality. Therefore,the wavelength dispersion must be compensated.

For a dispersion compensation method, a method for restoring waveformdistortion by inserting a device having a dispersion characteristic theopposite of an optical fiber (dispersion compensation fiber) in atransmission line is used. Furthermore, variable dispersion compensatorshave been developed which incorporate a chromatic dispersion generationdevice (VIPA) and a light-returning device (non-spherical mirror) inorder to cope with the change of the dispersion characteristic due tothe temperature, the pressure and the like of an optical fiber (JapanesePatent Applications 10-534450 and 11-513133).

FIG. 1 shows the basic configuration of a variable dispersioncompensator using a VIPA.

Beams inputted from a fiber 10 are collected in a form of a line or dotsby a lens 11 and are inputted to a VIPA 12. The VIPA is a transparentparallel plate on both sides of which a reflection film is formed.Although one reflection film has a reflectance of 100%, the other has areflectance of less than 100%, and typically of 95%. Therefore, beamsinputted to the VIPA 12 are repeatedly reflected between thesereflection films and some of the beams are repeatedly outputted to theoutside at one time from a surface with a low reflectance. Since thebeams are outputted to the outside at each reflection, the beams havephase differences between each other. Therefore, if the beams interferewith each other, beams with a prescribed wavelength are formed intoluminous flux that propagates in a prescribed direction. Thus, the VIPA12 is a device for generating a plurality of pieces of luminous fluxthat propagates in different directions depending on the wavelengths.

The outputted beams are collected at a lens 13 and are reflected on anon-spherical mirror 14. In this case, as shown by dotted lines in FIG.1, if attention is focused on one beam of the luminous flux, byreflecting a specific beam on the non-spherical mirror and by changingthe input position after return from the output position from the VIPAwhen returning the beam to the VIPA 12, the distance between the lens 11and fiber 10 that the beam propagates can be changed. Specifically, thepropagation distance can be extended, and propagation delay can beapplied to the beam. If a plurality of beams with different wavelengthstake different routes, the respective propagation delay of the beams canbe changed by a wavelength, and wavelength dispersion can be generated,accordingly. If the wavelength dispersion of an optical fiber iscompensated, a reciprocal dispersion having a reverse characteristic ofcanceling the wavelength dispersion of the beam is applied to the beam.

This compensator has a characteristic of freely changing a compensationamount by moving the non-spherical mirror depending on a dispersionvalue. The non-spherical mirror has a gradation structure, such as aconcave surface and a convex surface.

FIG. 2 shows a non-spherical mirror.

This non-spherical mirror is located on a moving stage. If the mirror ismoved in the direction of an arrow shown in FIG. 2, the shape of thelight input position of the mirror changes. Therefore, a plurality ofdifferent chromatic dispersions (wavelength dispersions) can begenerated.

FIG. 3 shows the wavelength dispersion and signal degradation of atransmission line, and the compensation.

For example, as shown in FIG. 3, if an input pulse (1) is transmittedfrom a transmitter and is received by a receiver through an opticalfiber, the pulse width of an output pulse (2) is expanded by wavelengthdispersion and the pulse is distorted. In this case, if a variabledispersion compensator (hereinafter a VIPA, for example) is inserted andreciprocal dispersion is given to the output pulse (2), the distortionof the pulse can be compensated. Therefore, the receiver can receive apulse without distortion (3).

FIG. 4 shows dispersion to be compensated by a VIPA.

If the wavelength of a pulse and the dispersion of an output pulse (3)are assumed to be λ0 and 100 ps/nm, respectively, the relationshipbetween the wavelength and dispersion becomes as shown in FIG. 4. Inthis case, dispersion compensation by a VIPA means the total dispersionamount is reduced to 0 ps/nm. Thus, a post-compensation pulse of 0 ps/nmis generated. Thus, the VIPA reduces the total dispersion amount to zeroby shifting the dispersion amount that a beam suffers from thepropagation through the optical fiber upward or downward (reciprocaldispersion).

<Problem No. 1>

According to the conventional method described above, the stage must bemoved depending on a dispersion compensation amount. Therefore, if thecompensation range is extended, the non-spherical mirror must be madelonger and a movement amount also increases. However, since the increasein a stage movement amount greatly affects the accuracy of the stagemovement, dispersion cannot be accurately compensated, which is aproblem.

Once a non-spherical mirror is designed, the mirror can compensate foronly a specific band. Therefore, in order to compensate for a new band,a new non-spherical mirror must be designed.

<Problem No. 2>

Although the conventional method gives a pulse with reciprocaldispersion, a case where this method is applied to a WDM beam isstudied.

FIG. 5 shows a case where the conventional wavelength dispersion methodis applied to a WDM beam.

In this case, it is assumed that there are three waves (λ1<λ0<λ2). Asshown in FIG. 5, λ1, λ0, and λ2 take different dispersion valuesdepending on the wavelengths, that is, a dispersion slope in an opticalfiber (curve 1). In this case, if the dispersion is shifted so that thedispersion value of λ0 becomes zero in a VIPA, as shown in FIG. 4, thedispersion values of λ1 and λ2 do not become zero. Since the VIPA shiftsthe curve by a specific dispersion amount throughout the entirewavelength, the VIPA simply shifts curve 1 upward or downward.Therefore, it is impossible to simultaneously reduce all the dispersionvalues of λ1, λ0 and λ2 to zero, which is also a problem.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a variablewavelength dispersion compensator for also compensating for a wavelengthdispersion slope.

The variable wavelength dispersion compensator of the present inventioncomprises an angular dispersion unit giving angular dispersion to aninput beam and a surface-shape variable mirror returning theangle-dispersed beam to the angular dispersion unit, the surface shapeof which can be changed. The compensator gives desired wavelengthdispersion to a beam by reflecting a beam inputted from the angulardispersion unit on the surface-shape variable mirror unit, inputting thereflected beam to the angular dispersion unit again, and outputting thebeam from the angular dispersion unit.

According to the present invention, since a surface-shape variablemirror, the surface shape of which can be changed, is used, differentlyfrom the conventional method, the mirror shape can be changed for eachwavelength, wavelength dispersion can be appropriately compensated and awavelength dispersion slope can also be appropriately compensated.

The variable wavelength dispersion compensator of the present inventioncan cope with a change in the wavelength dispersion characteristic of anoptical fiber due to deterioration caused by aging, a change in awavelength dispersion amount to be compensated due to the extension of atransmission line and the like, by changing the surface shape of themirror of the compensator without replacing the compensator itself.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the basic configuration of a variable dispersioncompensator using a VIPA;

FIG. 2 shows a non-spherical mirror;

FIG. 3 shows the wavelength dispersion and signal degradation of atransmission line and the compensation;

FIG. 4 shows dispersion to be compensated by a VIPA;

FIG. 5 shows a case where the conventional wavelength dispersion methodis applied to a WDM beam;

FIG. 6 shows the basic configuration of the preferred embodiment of thepresent invention;

FIGS. 7A and 7B show the operation of the surface-shape variable mirrorof the preferred embodiment;

FIG. 8 shows the configuration of the first preferred embodiment of thepresent invention;

FIG. 9 shows the configuration of the second preferred embodiment of thepresent invention;

FIG. 10 shows the effects of the configuration shown in FIG. 9;

FIG. 11 shows the detailed structure of a variable mirror (No. 1);

FIG. 12 shows the detailed structure of a variable mirror (No. 2);

FIG. 13 shows the configuration of the third preferred embodiment of thepresent invention; and

FIG. 14 shows the structure of a piezo stage.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 6 shows the basic configuration of the preferred embodiment of thepresent invention.

In this preferred embodiment, a wavelength dispersion compensator usinga VIPA uses a mirror, the surface shape of which can be changed, as anon-spherical mirror.

The surface-shape variable mirror shown in FIG. 6 comprises a thinmirror, piezo stages and pins. The pin is attached to the tip of thepiezo stage, and this pin and the mirror face are joined.

FIGS. 7A and 7B show the operation of the surface-shape variable mirrorof this preferred embodiment.

Since the piezo stage is expandable, for example, if only the middlestage is squeezed, as shown in FIG. 7A, a concave mirror face can beformed. This corresponds to the section (1) of the non-spherical mirrorshown in FIG. 2. If the upper and lower stages are squeezed and themiddle stage is expanded, the section shown in FIG. 7B is obtained. Thiscorresponds to the section (3) shown in FIG. 2.

Specifically, the surface-shape variable mirror comprises one mirror anda plurality of piezo stages. By expanding/squeezing the plurality ofpiezo stages, a variety of mirror shapes can be formed. Therefore, thereis no need to prepare all necessary shapes in advance, unlike anon-spherical mirror, and only one mirror can produce any desired shape.

Since the piezo stage can be controlled in units of several nanometers,a fine surface shape can be produced.

Although in this preferred embodiment, a thin mirror is used, forexample, the mirror can be produced by evaporating gold onto a thinglass plate yielding a thickness of approximately 100 μm. Basically, itis sufficient if both the glass plate and the evaporated gold havesufficient thickness to not break when the piezo stage isexpanded/squeezed and if the surface is a mirror.

Problem No. 1 can be solved by using the surface-shape variable mirrordescribed above. Specifically, even if a compensation band is expanded,it is sufficient to change a mirror shape by appropriately collectingbeams on a lens and changing the movement amount of the piezo stage.Therefore, there is no problem of accuracy degradation accompanying bothan increase of mirror length and an increase of a stage movement amount.Even if a compensation range is modified, there is no need to prepare anew mirror and the problem can be easily solved by transforming a mirrorsurface shape.

Problem No. 2 can be solved as follows. By separating WDM beams for eachwavelength using a diffraction grid and the like, applying this mirrorfor each wavelength and optimally compensating for dispersion for eachwavelength, the dispersion of each wavelength can be reduced to zero.

FIG. 8 shows the configuration of the first preferred embodiment of thepresent invention.

The first preferred embodiment can be implemented by replacing thenon-spherical mirror and moving stage with a surface-shape variablemirror.

Beams inputted from a fiber 10 are collected at a VIPA 12 by a lens 11and are outputted as a plurality of pieces of different flux for eachwavelength. The outputted beams are collected at a surface-shapevariable mirror 20 by a lens 13. A piezo stage 21 is provided at theback of the surface-shape variable mirror 20, and the mirror face of thesurface-shape variable mirror 20 can be transformed into an arbitraryshape. Prescribed wavelength dispersion can be generated by calculatingwavelength dispersion to be generated by the VIPA 12 and determining theshape of the mirror face so as to generate desired dispersion.

FIG. 9 shows the configuration of the second preferred embodiment.

In this preferred embodiment, the output beams from the VIPA 12 arebranched for each wavelength using a diffraction grid, and the branchedbeams are collected at a plurality of surface-shape variable mirrors(variable mirrors 1-3). Beams can be branched into, for example, threegroups of λ1, λ0, and λ2 (λ1<λ0<λ2) shown in FIG. 9 for each wavelengthby using a diffraction grid. Although in this preferred embodiment, adiffraction grid 25 is used to branch beams for each wavelength,anything that causes the dispersion of a wavelength, such as a prism,can be used.

Each group of beams with a different wavelength is collected at adifferent point through a lens. In FIG. 9, λ1, λ0, and λ2 are collectedat a surface-shape variable mirror 1 (variable mirror 1), asurface-shape variable mirror 2 (variable mirror 2), and a surface-shapevariable mirror 3 (variable mirror 3), respectively.

The surface-shape variable mirrors 1-3 (variable mirrors 1-3) canproduce different surface shapes. Therefore, a different dispersionvalue can be given to each of λ1, λ0, and λ2.

FIG. 10 shows the effects of the configuration shown in FIG. 9.Specifically, as shown in FIG. 10, if a different dispersion is givenfor each wavelength using the surface-shape variable mirror of thispreferred embodiment when dispersion before compensation is as shown bythe broken line, after compensation, all dispersion values can bereduced to zero, as shown by the solid line. Therefore, the dispersionslope of a WDM beam can also be compensated.

Although in FIG. 10, a plurality of surface-shape variable mirrors arelocated separately and in parallel, there is no need to separate theplurality of mirrors. For example, a piezo stage can also betwo-dimensionally located against one large mirror.

FIGS. 11 and 12 show the detailed structure of such a variable mirror.

For such a variable mirror, for example, a glass plate (silica systemglass) having both the thickness of 100 μm and the outer dimensions of10×3 mm is used. FIGS. 11 and 12 show the longer side section of theglass plate. A glass plate having some hardness, the elasticity of whichcan be changed, and which cannot be broken is used. Each end of themirror is mechanically fixed. This joint and the piezo are fixed bysoldering and the like. The pin and the back of the mirror are alsofixed by soldering and the like. It is assumed that the mirror is usedin a state where there is a small displacement amount and there is noplasticity transformation in the solder layer, and in realty the mirroris used in a state where the solder will not be broken off.

Although in FIG. 11, the position of the middle piezo stage is differentfrom those of the upper and lower piezo stages, the mirror is designedso that both the pin and solder can be easily seen. The middle piezostage is located in such a way that the distance between the variablemirror and middle piezo stage becomes the same as both the distancebetween the variable mirror and upper piezo stage and the distancebetween the variable mirror and lower piezo stage. Furthermore, althoughin FIG. 11, one middle piezo stage is provided, in reality, two or moremiddle piezo stages can also be provided. By providing many piezostages, a mirror face with a more complex shape can be produced.

On the mirror face of the glass plate, gold and the like is plated. Thethickness of the plating is thin enough to be sufficient for theelasticity to change as the glass plate is transformed and is thickenough to be sufficient for the plating to be prevented from tearing offas the elasticity of the glass plate changes. The supporting table ofthe piezo stages, which is not shown in FIG. 11, is not especiallylimited if the piezo stages can be properly fixed at a prescribedposition.

As shown in FIG. 12, the piezo stage has one shaft. In FIG. 11, thepiezo stage moves in the vertical direction against the mirror face(horizontal direction against a paper surface). For example, if the pinof the middle piezo stage is expanded, as shown in FIG. 12, a convexmirror face can be formed. Conversely, if the pin of the middle piezostage is squeezed, a concave mirror face is formed.

Although in FIGS. 11 and 12, only one middle piezo stage is provided, iftwo or more piezo stages are provided and alternately expanded/squeezed,a wavy mirror face can also be formed. In this way, by providing two ormore piezo stages, a more complex mirror face can be formed.

FIG. 13 shows the configuration of the third preferred embodiment of thepresent invention.

In the preferred embodiment shown in FIG. 9, a plurality of separatevariable mirrors are provided to compensate for wavelength dispersionwith a plurality of wavelengths and to eliminate the influence of adispersion slope from beams. In this preferred embodiment, atwo-dimensional variable mirror 30 obtained by incorporating thesemirrors is used instead of the plurality of separate variable mirrors.

Beams inputted to a fiber 10 are branched for each wavelength by adiffraction grid 25 after passing through both a lens 11 and a VIPA 12and after being angular-dispersed. In this example, the beams arebranched into three groups of beams with wavelengths λ1, λ0 and λ2. Inthe two-dimensional variable mirror 30, piezo stages 31 aretwo-dimensionally located and a mirror face 32 can be transformed morecomplexly. Thus, the mirror face 32 are continuously transformed up to aposition where beams with wavelength λ0 hit the mirror face and theposition where beams with wavelength λ0 hit the mirror face has a shapesuited for appropriately compensating for the dispersion of beams withwavelength λ0. Similarly, the mirror face 32 are continuouslytransformed up to a position where beams with wavelength λ2 hit themirror face and the position where beams with wavelength λ2 hit themirror face has a shape suited for appropriately compensating for thedispersion of beams with wavelength λ2.

The number of piezo stages 31 is increased and a more complex mirrorface can thus be formed. Then, one mirror face can compensate for aplurality of beam groups with a plurality of wavelengths.

FIG. 14 shows the structure of a piezo stage.

The piezo stage comprises a piezo stack having a pin at the tip. Thepiezo stack is cased. The piezo stack has a structure where a pluralityof ceramic disks are piled sandwiching electrodes. If a voltage isapplied to the electrode, the ceramic disks expand/contract. Then, theexpansion/contraction of the ceramic disks moves the pin attached at thetip of the piezo stack. A power cable is connected to the case thatencloses a piezo stack, and voltage can be applied to the electrode,accordingly.

Although in the description of this preferred embodiment, a VIPA isdescribed as one example of a device for angular-dispersing inputtedbeams, according to the present invention, the device is not limited toa VIPA. The combination of a transmission type diffraction grid and areflection type diffraction grid can also be used to perform a functionequivalent to a VIPA. Similarly, the surface-shape variable mirror isnot limited to the combination of a glass plate and piezo stages, whicha person having an ordinary skill in the art can easily understand.

By adopting the surface-shape variable mirror of the present inventiondescribed above, a variety of mirror face shapes can be formed.Therefore, the problems, such as the degradation of compensationaccuracy and the reproduction of a mirror, can be solved.

Furthermore, dispersion slope compensation can also be implemented,which was impossible by the conventional method.

Even if a dispersion compensation amount to be compensated changes whena new optical fiber is laid, a wiring extension installation is carriedout or a repeater is incorporated, and the dispersion compensator of thepresent invention can cope with such a situation only by modifying themirror face, and there is no need for another dispersion compensator.Even if a dispersion compensation amount to be compensated changes dueto the deterioration caused by aging of an optical fiber and the like,similarly the situation can be coped with only by transforming themirror face.

1. A reflector unit, comprising: an angular dispersion unit angular dispersing input beams; a mirror unit returning the angular dispersed beams to the angular dispersion unit; and a control unit changing a surface shape of the mirror unit within a surface region of the mirror unit.
 2. The reflector unit according to claim 1, wherein the control unit is provided at the back of the mirror unit.
 3. The reflector unit according to claim 2, wherein the control unit changes the surface shape of the mirror unit by transforming the shape of the mirror unit into an arbitrary shape to generate a dispersion based on a wavelength dispersion generated by the angular dispersion unit and the surface shape of the mirror unit.
 4. The reflector unit according to claim 1, wherein the control unit comprises a shaft moving in a vertical direction against a back of the mirror unit.
 5. A reflector unit, comprising: an angular dispersion unit angular dispersing input beams; mirror units returning the angular dispersed beams to the angular dispersion unit; a branching unit branching the angular dispersed beams into beam groups with different wavelengths; and a control unit changing corresponding surface shapes of the mirror units within corresponding surface regions of the mirror units.
 6. The reflector unit according to claim 5, wherein each surface shape is set where wavelength dispersion is compensated for each branched beam group.
 7. The reflector unit according to claim 5, wherein the control unit changes the corresponding surface shapes of the mirror units by transforming the corresponding shapes of the mirror units into arbitrary shapes to generate a dispersion based on a wavelength dispersion generated by the angular dispersion unit and the corresponding surface shapes of the mirror units.
 8. The reflector unit according to claim 5, wherein said branching unit is a diffraction grid or a chromatic dispersion generation device.
 9. A reflector unit, comprising: an angular dispersion unit angular dispersing input beams; a two-dimensional mirror unit returning the angular dispersed beams to the angular dispersion unit; and control units two-dimensionally located with respect to the two-dimensional mirror unit changing a surface shape of the two-dimensional mirror unit in two-dimensional directions within a surface region of the two-dimensional mirror unit.
 10. The reflector unit according to claim 9, further comprising: a branching unit branching the angular dispersed beams into beam groups with different wavelengths.
 11. The reflector unit according to claim 10, wherein the surface shape of the two-dimensional mirror unit changes in two-dimensional directions so that each branched beam group is received on a part of the surface and a prescribed wavelength dispersion is given to the branched beam group.
 12. The reflector unit according to claim 9, wherein said branching unit is a diffraction grid or a chromatic dispersion generation device. 